1. Field of the Invention
The present invention relates to the cracking of hydrocarbons that contain relatively non-volatile hydrocarbons and other contaminants.
2. Description of Background and Related Art
Steam cracking has long been used to crack various hydrocarbon feedstocks into olefins. Conventional steam cracking utilizes a pyrolysis furnace which has two main sections: a convection section and a radiant section. The hydrocarbon feedstock typically enters the convection section of the furnace as a liquid (except for light feedstocks which enter as a vapor) wherein it is typically heated and vaporized by indirect contact with hot flue gas from the radiant section and by direct contact with steam. The vaporized feedstock and steam mixture is then introduced into the radiant section where the cracking takes place. The resulting products including olefins leave the pyrolysis furnace for further downstream processing, such as quenching.
Conventional steam cracking systems have been effective for cracking high-quality feedstock which contain a large fraction of light volatile hydrocarbons, such as gas oil and naphtha. However, steam cracking economics sometimes favor cracking lower cost heavy feedstocks such as, by way of non-limiting examples, crude oil and atmospheric resid. Crude oil and atmospheric resid contain high molecular weight, non-volatile components with boiling points in excess of 1100° F. (590° C.). The non-volatile, components of these feedstocks lay down as coke in the convection section of conventional pyrolysis furnaces. Only very low levels of non-volatile components can be tolerated in the convection section downstream of the point where the lighter components have fully vaporized. Additionally, during transport some naphthas are contaminated with heavy crude oil containing non-volatile components. Conventional pyrolysis furnaces do not have the flexibility to process resids, crudes, or many resid or crude contaminated gas oils or naphthas which are contaminated with non-volatile components hydrocarbons.
To solve such coking problem, U.S. Pat. No. 3,617,493, which is incorporated herein by reference, discloses the use of an external vaporization drum for the crude oil feed and discloses the use of a first flash to remove naphtha as vapor and a second flash to remove vapors with a boiling point between 450 to 1100° F. (230 to 600° C.). The vapors are cracked in the pyrolysis furnace into olefins and the separated liquids from the two flash tanks are removed, stripped with steam, and used as fuel.
U.S. Pat. No. 3,718,709, which is incorporated herein by reference, discloses a process to minimize coke deposition. It provides preheating of heavy feedstock inside or outside a pyrolysis furnace to vaporize about 50% of the heavy feedstock with superheated steam and the removal of the residual, separated liquid. The vaporized hydrocarbons, which contain mostly light volatile hydrocarbons, are subjected to cracking.
U.S. Pat. No. 5,190,634, which is incorporated herein by reference, discloses a process for inhibiting coke formation in a furnace by preheating the feedstock in the presence of a small, critical amount of hydrogen in the convection section. The presence of hydrogen in the convection section inhibits the polymerization reaction of the hydrocarbons thereby inhibiting coke formation.
U.S. Pat. No. 5,580,443, which is incorporated herein by reference, discloses a process wherein the feedstock is first preheated and then withdrawn from a preheater in the convection section of the pyrolysis furnace. This preheated feedstock is then mixed with a predetermined amount of steam (the dilution steam) and is then introduced into a gas-liquid separator to separate and remove a required proportion of the non-volatiles as liquid from the separator. The separated vapor from the gas-liquid separator is returned to the pyrolysis furnace for heating and cracking.
The present inventors have recognized that in using a flash to separate heavy liquid hydrocarbon fractions from the lighter fractions which can be processed in the pyrolysis furnace, it is important to effect the separation so that most of the non-volatile components will be in the liquid phase. Otherwise, heavy, coke-forming non-volatile components in the vapor are carried into the furnace causing coking problems.
The present inventors have also recognized that in using a flash to separate non-volatile components from the lighter fractions of the hydrocarbon feedstock, which can be processed in the pyrolysis furnace without causing coking problems, it is important to carefully control the ratio of vapor to liquid leaving the flash. Otherwise, valuable lighter fractions of the hydrocarbon feedstock could be lost in the liquid hydrocarbon bottoms or heavy, coke-forming components could be vaporized and carried as overhead into the furnace causing coking problems.
The control of the ratio of vapor to liquid leaving flash has been found to be difficult because many variables are involved. The ratio of vapor to liquid is a function of the hydrocarbon partial pressure in the flash and also a function of the temperature of the stream entering the flash. The temperature of the stream entering the flash varies as the furnace load changes. The temperature is higher when the furnace is at full load and is lower when the furnace is at partial load. The temperature of the stream entering the flash also varies according to the flue gas temperature in the furnace that beats the feedstock. The flue-gas temperature in turn varies according to the extent of coking that has occurred in the furnace. When the furnace is clean or very lightly coked, the flue-gas temperature is lower than when the furnace is heavily coked. The flue-gas temperature is also a function of the combustion control exercised on the burners of the furnace. When the furnace is operated with low levels of excess oxygen in the flue gas, the flue gas temperature in the mid to upper zones of the convection section will be lower than that when the furnace is operated with higher levels of excess oxygen in the flue-gas. With all these variables, it is difficult to control a constant ratio of vapor to liquid leaving the flash.
The present invention offers an advantageously controlled process to optimize the cracking of volatile hydrocarbons contained in the heavy hydrocarbon feedstocks and to reduce and avoid the coking problems. The present invention provides a method to maintain a relatively constant ratio of vapor to liquid leaving the flash by maintaining a relatively constant temperature of the stream entering the flash. More specifically, the constant temperature of the flash stream is maintained by automatically adjusting the amount of a fluid stream mixed with the heavy hydrocarbon feedstock prior to the flash. The fluid optionally is water.
The present invention also provides a method to maintain a relatively constant hydrocarbon partial pressure of the flash stream. The constant hydrocarbon partial pressure is maintained by controlling the flash pressure and the ratio of fluid and steam to the hydrocarbon feedstock.
Separate applications, one entitled “CONVERTING MIST FLOW TO ANNULAR FLOW IN THERMAL CRACKING APPLICATION,” U.S. application Ser. No. 10/189,618, filed Jul. 3, 2002, and one entitled “PROCESS FOR CRACKING HYDROCARBON FEED WITH WATER SUBSTITUTION”, U.S. application Ser. No. 10/188,901, filed Jul. 3, 2002, are being concurrently filed herewith and are incorporated herein by reference.